Method of Operating a Rotatable Part

ABSTRACT

One embodiment of the present patent application is a method of operating a rotatable part. The method includes providing a system including a rotatable part, a magnet, an emf generating circuit, a processor, and a transmitter. The magnet and the emf generating circuit are mounted for relative rotational motion there between while the rotatable part is rotating. The processor is connected to receive a signal derived from the emf generating circuit and provide data for transmission by the transmitter. The method further includes rotating the rotatable part and generating an emf in the emf generating circuit while the rotatable part rotates. The method also includes using a signal derived from the emf to acquire data indicating a change in a mechanical property of the rotatable part while the rotatable part is rotating in which the data indicating a change in the mechanical property indicates a condition for maintenance The method further includes transmitting information related to the data with the transmitter and providing maintenance to the system based on the information.

RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.10/769,642, filed Jan. 31, 2004, and claims priority of thatapplication. The 10/769,642 application is a continuation-in-part ofU.S. patent application Ser. 10/379,223, filed Mar. 5, 2003 which claimspriority of U.S. Provisional Patent Application No. 60/362,432, fileMar. 7, 2002 and No. 60/443,120, filed Jan. 28, 2003.

This patent application is related to the following US patentapplications:

09/731,066, docket number 1024-034, filed Dec. 6, 2000, incorporatedherein by reference;

09/757,909, docket number 1024-035, filed Jan. 10, 2001, incorporatedherein by reference;

09/801,230, docket number 1024-036, filed Mar. 7, 2001, incorporatedherein by reference;

09/768,858, docket number 1024-037, filed Jan. 24, 2001, incorporatedherein by reference;

09/114,106, docket number 1024-041, filed Jul. 11, 1998, incorporatedherein by reference;

09/457,493, docket number 1024-045, filed Dec. 8, 1999, incorporatedherein by reference;

60/362,432, docket number 115-004, filed Mar. 7, 2002, incorporatedherein by reference;

60/443,120 docket number 115-008, filed Jan. 28, 2003, incorporatedherein by reference;

10/379,224, docket number 115-004, filed Mar. 5, 2003 incorporatedherein by reference; and 10/379,223, docket number 115-008, filed Mar.5, 2003, incorporated herein by reference.

FIELD

This patent application generally relates to collecting and transmittingdata. More particularly, it relates to a device for sensing, storing andtransmitting data. Even more particularly, it relates to a device thatcan that can be attached to a structure or live subject and that canharvest energy from its environment to power sensing, storing andtransmitting data about the structure or live subject.

BACKGROUND

Several available devices convert mechanical energy in the localenvironment into electrical energy, including the Seiko “Kinetic” watchand mechanical wind-up radios. An article, “Energy Scavenging withShoe-Mounted Piezoelectrics,” by N. S. Shenck. and J. A. Paradisoreports on systems that capture energy from the user's environment toprovide electricity to wearable microelectronic devices withoutbatteries. The unobtrusive devices scavenge electricity from the forcesexerted on a shoe during walking The devices include a flexiblepiezoelectric foil stave to harness sole-bending energy and a reinforcedpiezoelectric dimorph to capture heel-strike energy. They also report onprototype development of radio frequency identification (RFID) tagswhich are self powered by a pair of sneakers. A recent report by Menigeret al., entitled “Vibration-to-Energy Conversion”, discloses amicroelectromechanical system (MEMs) device for the conversion ofambient mechanical vibration into electrical energy through the use of avariable capacitor. However, these MEMs systems only demonstrated 8microwatts of power. Transmission of RF data over distances of 20 feetor more requires milliwatt power levels.

Low power sensors have been developed, as described on commonly assignedU.S. patent application Ser. No. 09/731,066, to Arms, that includes asensing unit for attaching to a structure or live subject for sensing aparameter of the structure or live subject. The sensing unit includes asensor, a data storage device, and a transmitting device. The datastorage device is for storing data from the sensor. Power is provided bya power supply such as a rechargeable battery or fuel cell. Therechargeable battery can be recharged by inductive coupling from anexternal control unit.

Over the past years, sensors, signal conditioners, processors, anddigital wireless radio frequency (RF) links have become smaller,consumed less power, and included higher levels of integration. The09/731,066 application, for example, provides sensing, acquisition,storage, and reporting functions. Wireless networks coupled withintelligent sensors and distributed computing have enabled a newparadigm of machine monitoring.

A paper, “Wireless Inductive Robotic Inspection of Structures,” byEsser, et al, proceedings of the IASTED International Conference,Robotics and Applications 2000, Aug. 14-16, 2000, Honolulu, Hi.,describes an autonomous robotic structural inspection system capable ofremote powering and data collection from a network of embedded sensingnodes and providing remote data access via the internet. The system usesmicrominiature, multichannel, wireless programmable addressable sensingmodules to sample data from a variety of sensors. The nodes areinductively powered, eliminating the need for batteries orinterconnecting lead wires.

Wireless sensors have the advantage of eliminating the cost ofinstalling wiring. They also improve reliability by eliminatingconnector problems. However, wireless sensors still require system powerin order to operate. If power outages occur, critical data collected bythe sensors may be lost. In some cases, sensors may be hardwired to avehicle's power system. In other cases however, the need to hard wire toa power system defeats the advantages of wireless sensors, and this maybe unacceptable for many applications. Most prior wireless structuralmonitoring systems have therefore relied on continuous power supplied bybatteries. For example, in 1972, Weiss developed a battery poweredinductive strain measurement system, which measured and counted strainlevels for aircraft fatigue. Traditional batteries, however, becomedepleted and must be periodically replaced or recharged, adding anadditional maintenance task that must be performed. This is particularlya problem for monitors used for a condition based maintenance programsince it adds additional maintenance for the condition based monitoringsystem itself.

None of the systems for sensing changes in the environment havecollected available mechanical energy to provide the electricity forrunning the sensors, storing data from the sensor, or communicating thedata externally. Thus, a better system for powering sensors and storagedevices, and for transmitting data gathered by sensors is needed, andthis solution is provided by the following.

SUMMARY

One aspect of the present patent application is a method of operating arotatable part. The method includes providing a system including arotatable part, a magnet, an emf generating circuit, a processor, and atransmitter. The magnet and the emf generating circuit are mounted forrelative rotational motion there between while the rotatable part isrotating. The processor is connected to receive a signal derived fromthe emf generating circuit and provide data for transmission by thetransmitter. The method further includes rotating the rotatable part andgenerating an emf in the emf generating circuit while the rotatable partrotates. The method also includes using a signal derived from the emf toacquire data indicating a change in a mechanical property of therotatable part while the rotatable part is rotating in which the dataindicating a change in the mechanical property indicates a condition formaintenance The method further includes transmitting information relatedto the data with the transmitter and providing maintenance to the systembased on the information.

Another aspect of the present patent application is a method ofoperating a rotatable part. The method includes providing a systemincluding a rotatable part, a magnet, an emf generating circuit, aprocessor, and a transmitter. The magnet and the emf generating circuitare mounted for relative rotational motion there between while therotatable part is rotating. The processor is connected to receive asignal derived from the emf generating circuit and provide data fortransmission by the transmitter. The method further includes rotatingthe rotatable part and generating an emf in the emf generating circuitwhile the rotatable part is rotating. The method also includes using asignal derived from the emf to acquire data indicating a change in amechanical property of the rotatable part while the rotatable part isrotating in which the change in the mechanical property is related tooperation of said rotatable part. The method further includestransmitting information related to the data with the transmitter andadjusting operation of said rotatable part based on the information.

Another aspect of the present patent application is a method ofoperating a rotatable part. The method includes providing a rotatablepart, a magnet, an emf generating circuit, a circuit for converting analternating current into a direct current, a processor, and atransmitter. The processor has an active state and a sleep mode. Themagnet and the emf generating circuit are mounted for relativerotational motion there between while the rotatable part is rotating forgenerating an emf in the emf generating circuit. The circuit forconverting an alternating current to a direct current is connected toreceive an alternating current derived from the emf. The processor andthe transmitter are connected for receiving the direct current. Themethod further includes rotating the shaft, generating the emf,converting the emf to direct current, waking the processor, and drawingthe direct current to the processor to provide the processor in theactive mode. The method further includes drawing the direct current tothe transmitter and using the transmitter for transmitting data,shutting down power to the transmitter, and returning the processor tothe sleep mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a perspective view of an energy harvesting addressablewireless sensing node mounted on a machine or structure;

FIG. 1 b is a perspective view of components within the energyharvesting addressable wireless sensing node;

FIG. 1 c is a perspective view of the energy harvesting cantilever shownin FIG. lb with variable mass for tuning to a vibration frequency of themachine or structure;

FIG. 1 d is a schematic diagram of a base station for receiving saidwirelessly transmitted information;

FIG. 2 is an alternative embodiment in which a large sheet of PZT fiberis embedded in material, such as a hull of ship so vibration or strainenergy transmitted through the hull can be harvested;

FIG. 3 a, 3 b are block diagrams of one embodiment of an energyharvesting addressable wireless sensing node in which energy isharvested by a PZT;

FIG. 4 is a block diagram of an alternate embodiment of an energyharvesting addressable wireless sensing node in which energy isharvested from a power transmission line;

FIG. 5 is a block diagram of the wireless sensing module shown in FIGS.3 a, 3 b;

FIG. 6 a is a timing diagram of voltage across capacitor C2 of FIG. 11;

FIG. 6 b is a timing diagram of voltage across capacitor C1 of FIG. 11;

FIG. 6 c is a timing diagram of voltage across the transmitter of FIG.11 showing how charge gradually stored in long term storage capacitor C1is used to briefly power the transmitter or transceiver;

FIG. 7 is a cross sectional view of a tire having an energy harvestingdevice used to power transmitting pressure and temperature sense data;

FIG. 8 is a schematic diagram showing a receiver mounted in a vehiclethat receives signals indicating tire sensor data transmitted by each ofthe tires on the vehicle;

FIG. 9 is a diagram showing data from an experimental test showing thatthe PZT provided the same low current output as load resistance wasvaried from 100 ohms to 50 Kohms;

FIG. 10 is a diagram showing data from the experimental test showingthat the optimum load impedance, that delivers maximum power, was foundto be about 500 Kohms;

FIG. 11 a is a block diagram of an improved embodiment of an energyharvesting addressable wireless sensing node having an additional stageof charge storage, monitoring, switching, and impedance conversionbetween the rectifier and the controller of FIG. 3 a;

FIG. 11 b is a schematic diagram showing more detail than the blockdiagram of FIG. 11 a;

FIG. 12 is a schematic diagram showing a wireless web enabled sensornetwork (WWSN) system that requires very little power;

FIG. 13 is a block diagram of a shaft mounted energy harvesting system;

FIG. 14 is a schematic diagram of a portion of the system illustrated inFIG. 13 and showing the arrangement of rectifiers, a rate of rotationdetector, and a voltage regulator;

FIG. 15 is a three dimensional view of a shaft mounted energy harvestingsystem showing the system for harvesting energy from movement of coilson the shaft through a stationary magnetic field and placement of strainsensors on the shaft;

FIG. 16 is a block diagram of circuits used to adjust gain and offset ofsensor conditioning;

FIG. 17 is a flow chart representation of firmware running on themicroprocessor;

FIG. 18 is a block diagram of an embodiment of an energy harvestingaddressable wireless sensing node in which energy is harvested from arotating shaft, rectified, stored in an energy storage device, and thisenergy is switched in to power a transmitter when charge on the energystorage device reaches a threshold;

FIG. 19 is a block diagram of a network of systems; and

FIG. 20 is a block diagram of an embodiment showing feedback to a motorcontroller and the engine driving the shaft based on data transmittedfrom the shaft.

DETAILED DESCRIPTION

The present inventors recognized that substantial efficiency incollecting, storing, and transmitting data from wireless sensors couldbe provided by harvesting energy from the environment.

This patent application is aimed at developing a new class of sensingsystems that can wirelessly report data without the need for maintainingor replacing batteries. Instead, the sensing systems rely on harvestingvibration, strain energy, or magnetic coupled energy from the localenvironment for conversion to electrical power for storage and use tocollect, store, or transmit data by the sensing system. Thus, machines,structures, and live subjects can be monitored without the need forreplacing or recharging batteries or for a battery maintenance schedule.Truly smart structures and machines will thus be able to autonomouslyreport their condition throughout their operating life without themechanism used for reporting the data itself requiring maintenance Thesystem can be used to run and communicate with actuators as well assensors.

One important use of the present patent application is to improvetraditional condition based maintenance Condition based maintenanceprovides a more accurate and cost effective maintenance program forequipment or structures. The present patent application reducesunnecessary preventive maintenance for the devices used to monitor. Inaddition to providing for wireless communication without batteries, thepresent patent application provides the components necessary to realizethe potential benefits of condition based monitoring, includinginformation acquisition, analysis, storage, and reporting technologiesthat substantially lower power requirements, making energy harvestingfor condition based maintenance a realistic source of energy.

An illustration of condition based maintenance and another important useof an embodiment of this patent application is aboard ships wherebatteryless sensing systems may be used for wirelessly monitoring oildebris or oil condition, tank & hull corrosion, combustion pressure,water-lubricated-bearing wear, and machine condition. An embodiment canalso be used for integrated, hierarchical machinery diagnostics &prognostics; machinery diagnostics & prognostics; open systemsarchitecture condition based maintenance; human—computer interfacecondition based maintenance; and diagnostic of insulation, such as wireand windings. An embodiment can also be used on land vehicles oraircraft for purposes such as to monitor and report tire temperature andpressure. In each case mechanical energy, such as the energy ofvibration of the vehicle, can be used to power the sensor and itsstorage or communications apparatus.

Batteries, and the additional maintenance burden for replacing orrecharging batteries, are avoided by providing wireless sensing networksystems which can harvest energy from the local environment to providethe power needed for their own operation.

Numerous sources of ambient energy can be exploited for energyharvesting, including solar, wind, thermoelectric, water/wave/tide,rotation, strain, and vibration. For shipboard monitoring applicationsbelow deck and for monitoring tire pressure and temperature, mechanicalenergy harvesting devices, such as those that harvest strain orvibrational energy are preferred. In Navy applications, strain energywould be available on engine mounts, ship hull sections, and structuralsupport elements. Vibrational energy would be available on dieselturbine engine components, propeller shaft drive elements, and othermachinery and equipment. This energy could be harvested usingelectromagnetic devices (coil with permanent magnet), Weigand effectdevices, and piezoelectric transducer (PZT) materials. Of these, the PZTmaterials hold the most promise.

Recent developments in single crystal PZT have led to significantimprovements in the mechanical-to-electrical conversion coefficients(coupling coefficients), from 60% efficiency to 90% efficiency . Singlecrystals also exhibit higher operating strain capabilities thanconventional PZT materials (0.2% vs. 1.4%). These materials areavailable through TRS Ceramics (State College, Pa.).

Furthermore, PZT fibers have recently been made commercially availableat low cost for active damping of sporting equipment, such as baseballbats, tennis rackets, and skis (Advanced Cerametrics, Lambertville,N.J.). These fibers may be directly bonded to a straining element orstructure to generate electrical energy that can be harvested. A majoradvantage of these fiber piezoelectric materials is that they cantolerate the loss of many individual fibers in a bundle and stillfunction well. Since they are in mass production, they may be obtainedreadily and at relatively low cost. Because of these advantages thepresent patent application describes the use of these PZT materials forenergy harvesting wireless sensor networks. However, other devices andother sources of ambient energy can also be used.

The present inventors have used single crystal and PZT fibers to createworking energy harvesting prototypes that provide sufficient energy topower StrainLink wireless sensor transmitters available fromMicroStrain, Inc.

Energy harvesting addressable wireless sensing node 18 can be attachedto machine or structure 19 that is subject to vibration, as shown inFIG. 1 a. In one embodiment, PZT 20 is mounted to cantilever 22 whichcan be tuned with variable mass 24, as shown in FIGS. 1 b and 1 c, toprovide a device resonance frequency close to the vibration frequency ofmachine or structure 19, thereby optimizing energy harvesting. PZT 20can be either a crystal or a fiber. Cantilever 22 is mounted on PC board25 in enclosure 26.

Alternatively, a large sheet of PZT fiber 27 can be embedded in materialof hull 28 of ship 30 so vibration or strain energy transmitted throughhull 28 can be harvested, as shown in FIG. 2. Large sheets of PZT fiber27 are preferred because tuning is not readily available to harvest thestrain energy. A structure, such as hull 28 or the deck of a bridgebends only a limited amount, and the bending cannot be tuned as canflexural element by adjusting mass so as to take advantage of resonancefrequency to harvest more of the energy.

In the mechanical vibration embodiment, the source of mechanical energy,such as machine or structure 19, is converted to electrical energy inenergy harvesting addressable wireless sensing node 18′, which includesa miniature electric generator, such as PZT 20, as shown in FIG. 3 a. Aminiature electric generator can also be provided with a coil and magnetin relative rotational motion, as for example, would be available in thevicinity of spinning machinery or wheels.

Electrical power generated in PZT 20 is rectified in rectifier 40,stored in electrical storage device 42, and once sufficient energy hasbeen stored, is provided to a load, such as wireless sensing module 44,by means of controller 46.

In one experiment, a single crystal PZT 20 was connected to the circuitshown in FIGS. 3 a, 3 b, while vibration was applied to PZT 20. With aDC voltmeter across storage capacitor 42, upwards of 20 volts wasmeasured across the capacitor with approximately 0.08 inch deflection ofthe PZT 20 at a 50 Hz rate.

PZT 20 is inherently a high impedance device which provides a nearlyconstant current, so the storage capacitor charges linearly with time.Thus, the time for storage capacitor 42 to charge is found from T=CV/Iwhere C=capacitance value, V=voltage charged to, and I=the chargingcurrent.

The Microstrain StrainLink transmitter is also a constant current load,so calculating the discharge uses the same formula. A 47 uF cap chargedto 16 volts will supply 2.8 mA for 268 mS discharging to zero volts. Alow power StrainLink transmitter can be connected as load 44 in thecircuit of FIG. 3 a, 3 b and can run for 224 mS before reaching thereset voltage of 2.63 volts. This is enough time to transmit data fromseveral sensors. Obviously a bigger storage capacitance would supply alonger operating time as would any reduction in load current presentedby the transmitter. However, a longer time would be needed to charge alarger capacitor. Furthermore, the practicality of such a system isdependant on the continued availability of vibration input energy. Thus,the present device is ideally suited to applications where ambientvibration is continuous for long periods to provide for theself-discharge rate of storage capacitor 42, to provide power consumedby the circuit used to monitor charge and switch on the load, as well asto power the load.

In an alternative embodiment PZT 20 device could be replaced with coilwinding 47 a that is closely coupled to power transmission line 47 bthat would allow energy in the magnetic field around the transmissionline to be harvested, as shown in FIG. 4. Such a configuration could beused with thermocouples 47 c to measure the temperature of transmissionline 47 b and detect an overheated condition in transmission line 47 b.As with the PZT embodiment, the frequency of transmissions isproportional to current in the transmission line 47 b.

Wireless sensing module 44 includes microcontroller or microprocessor48, which controls provision of power to A/D converter 50, sensors 52,non-volatile memory 54, and RF transmitter 56, as shown in FIG. 5.Sensors can include such sensors as a temperature sensor, a straingauge, a pressure sensor, a magnetic field sensor, an accelerometer, ora DVRT. By selectively providing power to or withholding power fromthese devices microcontroller 48 can achieve substantial energy savings.Microcontroller 48 also controls flow of data from A/D converter 50,from sensors 52, to and from nonvolatile memory 54 and to RF transmitter56. A transceiver can be provided instead of RF transmitter 56 to enabletwo way communication, all powered by ambient vibrational energy.

The strain or vibrational energy 57 from the ambient environment isprovided to PZT transducer 20 mounted on a machine, structure, or livesubject, as shown in block diagram form in FIG. 3 a and in schematicform in FIG. 3 b. As indicated above, electrical output of PZT 20 isrectified in rectifier 40. DC output of rectifier 40 charges storagecapacitor 42. Controller 46 monitors charge stored on storage capacitor42, and when sufficient, provides Vcc power to wireless sensing module44 for transmitting sensor data through antenna 68 to receiver 69 a onbase station 69 b (FIG. 1 d). Receiver 69a can be a transceiver.Controller 46 includes monitoring device 70, and switch Q1, which isformed of MOSFET transistor 72. When voltage across capacitor 42 issufficient, monitoring device 70 turns on to provide Vcc to wirelesssensing module 44. To reduce leakage and ensure that wireless sensingmodule 44 remains fully off and does not load storage capacitor 42 whenvoltage across storage capacitor 42 is below a threshold, transistor 72is provided. When transistor 72 turns on, ground connection fromwireless sensing module 44 is established.

Transistor 72 is needed because when voltage provided by storagecapacitor 42 is too low, monitoring device 46 cannot provide its outputin a known state. Monitoring device 46 may turn on falsely and load downstorage device 42, preventing it from ever charging up. Monitoringdevice 46 is not itself a reliable switch unless supply voltage is abovea threshold. To provide for operation in the regime when supply voltageis below that threshold, switch 72 is provided to ensure that wirelesssensing module 44 remains fully off. Switch 72 connected betweenwireless sensing module 44 and ground and to has a single threshold.

In operation in one embodiment, monitoring device 70 becomes valid at1.8 volts. Switch Q1 transistor 72 turns on at 2.0 V, enabling wirelesssensing module 44 when storage capacitor 42 has sufficient charge tooperate monitoring device 70 properly and can hold it off. Finally, whenvoltage at storage capacitor 42 reaches 6.3 V monitoring device 70 turnson and transfers charge from storage capacitor 42 to power wirelesssensing module 44 for a brief period, until voltage discharges back to2.9 volts, at which point monitoring device 70 turns off furthertransfer, and monitoring device 70 therefore continues to be in a validstate for subsequent operation, well above the 1.8 volts level neededfor proper operation in a determinate state.

Thus, when sufficient charge is provided to storage capacitor 42 toprovide a voltage equal to a higher threshold, monitoring device 70turns on and connects wireless sensing module 44 to storage device 42.This discharges storage device 42 down to a lower threshold voltage atwhich point monitoring device 70 turns off, disconnecting wirelesssensing module 44 from storage device 62. Storage device 42 can thenrecharge from energy supplied from PZT 20. However, if storage device 42fully discharges, or if potential across storage device 42 falls belowthe lower threshold then monitoring device 70 may not be sufficientlypowered to operate correctly. It may not fully disconnect wirelesssensing module 44 from storage device 42, and thus, wireless sensingmodule 44 may continue to load storage device 42, preventing it fromever recharging. To prevent this possibility, switch 72 is providedwhich disconnects wireless sensing module 44 from ground when potentialacross storage capacitor 42 falls somewhat below the lower threshold.

The present inventors found that impedance mismatch between PZT 20 andstorage capacitor 42 limits the amount of power that can be transferredfrom PZT 20 to storage capacitor 42. They recognized that energytransfer was limited by the fact that AC power generated by PZT 20 ispresented by the PZT at a very high impedance and at low frequency. Theyobserved that PZT 20 behaves as a constant current source, and that whenpiezoelectric elements are used to charge capacitors, such as storagecapacitor 42, charging is determined by the short circuit currentcapability of PZT 20. When storage capacitor 42 is charged from aconstant current source, such as PZT 20, storage capacitor 42 willcharge at a rate proportional to the current provided by the constantcurrent source. They further recognized that since the current availablefrom PZT 20 is low, a long time is needed to charge a large capacitance,such as storage capacitor 42, needed for powering devices such aswireless sensing module 44 or other transmitters. They recognized thefurther difficulty that the larger leakage current presented by largercapacitors may exceed the charge rate of the constant current providedby PZT 20.

The present inventors developed a circuit that efficiently convertspower from a high impedance current source, such as PZT 20, to a lowimpedance voltage source capable of charging a capacitor or batterystorage device. The inventors also developed an efficient way todetermine when enough power has been accumulated and applying thataccumulated power to a useful purpose.

In addition, the present inventors recognized that if the availablepower in the piezoelectric element were to be efficiently converted fromits low current and high impedance current source to a voltage source,the capacitor could be charged much faster than if the same capacitorwere charged directly from the short circuit current of thepiezoelectric element without this conversion. For example, if a voltageconverter can present a 500K load to the piezoelectric element,approximately matching its impedance, the element will deliver 17.5volts at 35 uA or 610 microwatts. If this power was then converted downto 100 ohms source impedance, even at 80% efficiency, the charge currentwould be more than 2.2 mA. By comparison, the output at the same levelof excitation of the piezoelectric element when loaded to 100 ohmswithout a converter, is 6 millivolts at 60 uA or 0.36 microwatts, about1,700 times less power.

The inventors conducted empirical tests on a sample of piezoelectricmaterial in order to determine a viable topology of conversion circuit.A test was performed on a sample of highly flexible piezoelectric fiber.The sample was mounted in a 3 point bending jig with a strain gaugeattached to the material, and excited to the same strain levels at threedifferent frequencies. A decade resistance substitution box was used toload the output in order to determine the optimum load impedance formaximum power out of the material under these conditions. The same lowcurrent was measured as the load resistance was varied from 100 ohms to50 Kohms as shown in FIG. 9. The optimum load impedance, that deliversmaximum power, was found to be about 500 Kohms, as shown in FIG. 10.

The present inventors found that further substantial improvement inenergy harvesting is available by adding an impedance converter circuitto the circuit of FIG. 3 a that provide better impedance matching to thehigh impedance of PZT 20, while still finally providing the largecapacitance needed to power wireless sensing module 44. The improvementto energy harvesting addressable wireless sensing node 18″, illustratedin block diagram form in FIG. 11 a and in a schematic diagram in FIG. 11b, provides an additional stage of charge storage, monitoring,switching, and impedance conversion between rectifier 40 and controller46 of FIG. 3 a. In addition to providing more efficient transfer ofenergy from PZT to long term storage device 42′, the improvement allowsa much larger capacitor or a battery to be used for that long termstorage 42′, and this enables more information transfer by wirelesssensing module 44.

PZT 20 connected to a source of mechanical energy, such as vibration orstrain 57, produces a high impedance AC voltage in accordance with thestrain or vibration 57 applied to PZT element 20. D1 and D2 (FIG. 11 b)form Schottky barrier rectifier bridge rectifier 40 that converts the ACvoltage from PZT 20 to DC. PZT 20 charges reactance element 78, such assmall capacitor C2 along curve 80 until a voltage equal to Vth3 isreached, as shown in FIG. 11 a and FIG. 6 a.

Voltage Vth3 is sufficient to turn on switch 2, transistor 82 whichconnects DC-DC converter 84 to ground, enabling DC-DC converter 84 toturn on and operate. When DC-DC converter 84 turns on, it converts thehigh voltage stored on small capacitor C2 to a low voltage at a lowimpedance for providing a small boost 86 to the charge on long termstorage capacitor 42′, capacitor C1, as shown along charging curve 88 inFIG. 6B. While long term storage capacitor C1 is charging, smallcapacitor C2 is discharging. The discharge of small capacitor C2, isshown along curve 90 in FIG. 6 a, providing the charge to boost thevoltage of long term storage capacitor C1 by the small step 86 shown inFIG. 6 b. Voltage scales are the same on FIGS. 6 a, 6 b, 6 c. Smallcapacitor C2 continues to discharge through DC-DC converter 84 untilvoltage on small capacitor C2 equals voltage on long term storagecapacitor C1. Thus, as long term storage capacitor C1 charges up, smallcapacitor C2 discharges less and less fully, as shown by the continuousincrease in the discharge voltage level 92 in FIG. 6A with each chargingand discharging cycle of small capacitor C2, while the charge level oflong term storage capacitor C1 continuously increases.

Although voltage on small capacitor C2 discharges, second switch 82remains on because of delay introduced by capacitor C3 in parallel withresistor R2 in voltage divider 94. Thus, DC-DC converter 84 remains onwhile voltage across capacitor C2 drops below Vth3 as shown in FIG. 6A.R4, R5 and second switch 82 form a switch that disables any conversionuntil enough voltage is present on C2 to convert. This switch thresholdis set to approximately 22 volts. DC-DC converter 84 is a high frequencystepdown DC to DC converter that has a typical quiescent current of 12uA and is capable of 80% efficiency even with small load current. Inthis embodiment, DC-DC converter 84, U2 is an LT1934-1 (LinearTechnology, Milpitas, Calif.). This converter was chosen due to its verylow quiescent current.

As also described for the circuit of FIGS. 3 a and 3 b and the circuitof FIGS. 11 a and 11 b, resistors R1, R2, R3, and comparator U1 formmonitoring device 70 a and also form voltage sensitive switch 70 b thatturns off connection to load 44 until enough charge has been accumulatedon storage capacitor 42, 42′ to run load 44. Load 44 can be multiplewireless sensing module 44, or an array of such modules, as shown inFIG. 11 b. Monitoring device 70 a/voltage sensitive switch 70 b is in anundefined state, however, until at least 1.8 volts is available on itsVcc pin 7, which is connected to storage device 42, 42′. To avoidproblems from this undefined state, MOSFET switch Q1 is provided todisconnect load 44 until voltage on storage device 42, 42′ has reached2.0 volts. R2 & R3 set the turn-on threshold V_(th2) of voltagesensitive switch 70 b to 6.3 volts. R1 provides hysteresis to comparatorU1 giving it two thresholds. Voltage sensitive switch 70 b now turns onwhen voltage on storage device 42. 42′ reaches the higher thresholdV_(th1) of 6.3 volts and stays on until the voltage on storage device 42discharges down to V_(th2) the lower threshold of 2.9 volts. Whenstorage device 42, 42′ reaches its higher threshold of 6.3 volts thereis enough charge available on storage device 42, 42′ to power load 44 tooperate for a brief period, for example, to transmit a burst of data.Load 44 may be a StrainLink transmitter or a data logging transceiver.

None of the charge provided to long term storage device 42′, is suppliedto wireless sensing module 44 until the voltage on long term storagedevice 42′ reaches the higher threshold, V_(th1), as shown in FIG. 6B.When voltage on long term storage device 42′, C1 reaches V_(th1),monitoring device 70 now turns on, as described herein above. Switch Q1(transistor 72) has already turned on before V_(th2) was reached, andcharge is now transferred from long term storage device 42′, C1 tooperate wireless sensing module 44, as shown in FIGS. 6B and 6C. Voltageon long term storage device 42′, C1 discharges to V_(th2), about 2.9volts at which point monitoring device 70 turns off.

If voltage to switch Q1 declines too far, switch Q1 will turn off, andthis shuts off transmitter 44 until enough energy is accumulated instorage device 42′ to send another burst of data.

Multiple wireless sensing modules 44 or other devices can be provided ona network, each powered as described herein with energy harvested fromits environment. The multiple wireless sensing module 44 can transmit ondifferent frequencies or a randomization timer can be provided to add arandom amount of time after wake up to reduce probability of collisionsduring transmission. However, since the time for charging is likely todiffer from one wireless sensing module 44 to another, a randomizationtimer may not be needed. Each wireless sensing module 44 will transmitan address as well as data. Transceivers can be provided to eachwireless sensing module 44 to provide two way communication. Preferably,if a battery is used that is recharged from the environment, sufficientenergy will be available so it can wake up periodically to determine ifsomething is being transmitted to it. If not it can go back to sleepmode. If so, it can receive the transmission. All the members can bemanaged by a broadcast signal or each wireless sensing module 44 can beaddressed and programmed individually.

The present inventors have applied the energy harvesting system todesign a device for embedding in a tire by a tire manufacturer forharvesting energy and for monitoring parameters, such as tiretemperature and pressure on a vehicle and for transmitting the data, asshown in FIG. 7. The cross section of tire 100 shows the placement ofPZT 102, or several such PZT elements, on or within interior sidewall104 of tire 100 for gathering strain energy from flexing of tire 100 onrim 101 as the tire rotates. PZT 102 is connected to provide power toenergy harvesting addressable wireless sensing node 106 for transmittingdata from temperature and pressure sensors 108, such as SensorNor fromHorten, Norway, to report this tire data. Energy harvesting addressablewireless sensing node 106 can be programmed to provide it with a 128 bitaddress. With such a large address there are enough combinationspossible to allow every tire in the world to have a unique address.Thus, receiver 110 mounted in the vehicle can receive a signalindicating tire sensor data for each of the tires on the vehicle, asshown in FIG. 8. A display can provide the information to the operator.Alternatively, an alarm can signal when tire pressure or temperature isoutside specified limits. Interference from other vehicles can beavoided by displaying only data from tires having known addresses.

Local antennas 112 can be provided in each wheel well (not shown) andthe power output of energy harvesting addressable wireless sensing node106 can be adjusted to provide reliable communications within the wheelwell of the vehicle while avoiding interference with transmitters onadjacent vehicles.

Receiver 110, having antennas 112 positioned in each wheel well of thevehicle, can rapidly scan antennas 112 to determine the address andposition of each tire on the vehicle. Because of the scanning of theantennas, even if tires are rotated, the display can indicate thelocation of a tire having a problem. Most modern receivers have thecapability of accurately measuring received signal strength with fairlyhigh resolution. In the case of inner and outer wheels in a single wheelwell, these received signals can be qualified by received signalstrength indication to distinguish the tires in the wheel well, even ifthey are rotated. Thus, the tire further from the antenna will have theweaker signal strength. In addition, the serial numbers of each tirewould also be logged in the receiver flash memory to distinguish tireson the vehicle for feedback to a tire manufacturer.

One alternative to the tire position problem that does not requirescanning or multiple antennas, is to have a technician sequentially scana bar code on the tires at the time of tire installation on the vehicle,and communicate the tire position information to the automotivecommunications (CAN) bus or other communications bus within the vehicle,or even directly to the receiver. The position information is providedusing a different protocol than the information tires are sending sothis information can remain stored in the receiver while other dataabout the tire changes with each reading. In this way one receiveantenna could receive data and an identification code from all tires onthe vehicle, and the stored table linking identification and tireposition can be used to communicate the position, temperature, andpressure of each tire. Interference from transmitters on adjacentvehicles is avoided since they would not have known identificationcodes.

The present inventors have also found ways to reduce power consumptionas well as to provide power from energy harvesting. They recognized thatpower consumed by all of the system's components (sensor, conditioner,processor, data storage, and data transmission) must be compatible withthe amount of energy harvested. Minimizing the power required to collectand transmit data correspondingly reduces the demand on the powersource. Therefore, the present inventors recognized, minimizing powerconsumption is as important a goal as maximizing power generation.

The present inventors have developed and marketed sensors that requirevery little power. For example, they have previously reported onmicro-miniature differential variable reluctance transducers (DVRT's)capable of completely passive (i.e., no power) peak strain detection.These sensors can be embedded in a material and will continuouslymonitor for the existence of a damaging strain state. By providing ahermetic seal the sensors can withstand harsh environmental conditions(moisture, salt, and vibration). The sensors can be reset remotely usingshape memory alloys and (remotely applied) magnetic field energy, asdescribed in a copending patent application Ser. No. 09/757,909, docketnumber 1024-035, incorporated herein by reference. The present inventorshave also recently developed totally passive strain accumulationsensors, which can be used to monitor fatigue. Furthermore, they havedemonstrated novel radio frequency identification (RFID) circuits withthe capability of interrogating these sensors in under 50 microsecondsusing less than 5 microamperes of current. Thus, although small amountsof energy may be available from energy harvesting, the energy socollected is enough to power sensors, electronics, and transmitters.

The present inventors have also developed wireless web enabled sensornetwork (WWSN) systems that require very little power. One strategy forminimizing power is demonstrated by the WWSN network architectureillustrated in FIG. 12. This is an ad hoc network that allows thousandsof multichannel, microprocessor controlled, uniquely addressed sensingnodes TX to communicate to a central, Ethernet enabled receiver RX withextensible markup language (XML) data output format. A time divisionmultiple access (TDMA) technique is used to control communications. TDMAallows saving power because the nodes can be in sleep mode most of thetime. Individual nodes wake up at intervals determined by arandomization timer, and transmit bursts of data. By conserving power inthis manner, a single lithium ion AA battery can be employed to reporttemperature from five thermocouples every 30 minutes for a period offive years. The XML data format has the advantage of allowing any useron the local area network (LAN) to view data using a standard Internetbrowser, such as Netscape or Internet Explorer. Furthermore, a standard802.11b wireless local area network (WLAN) may be employed at thereceiver(s) end in order to boost range and to provide bi-directionalcommunications and digital data bridging from multiple local sensingnetworks that may be distributed over a relatively large area (miles).Further information about a wireless sensor network system developed bythe present inventors is in patent application docket number 115-004,incorporated herein by reference.

Another strategy for creating low power wireless sensor networks isdemonstrated by MicroStrain's Data Logging Transceiver network asdescribed in copending U.S. patent application Ser. No. 09/731,066,docket number 1024-034, incorporated herein by reference. This systememploys addressable sensing nodes which incorporate data loggingcapabilities, and a bi-directional RF transceiver communications links.A central host orchestrates sample triggering and high speed logging toeach node or to all nodes. Data may be processed locally (such asfrequency analysis) then uploaded when polled from the central host. Byproviding each sensor node with a 16 bit address, as many as 65,000multichannel nodes may be hosted by a single computer. Since each nodeonly transmits data when specifically requested, the power usage can becarefully managed by the central host.

For further energy savings, only limited data collected by sensors maybe transmitted. For example, minimum, maximum and average data can betransmitted to reduce the amount of data transmitted and to thereby saveenergy. Standard deviation can also be locally calculated andtransmitted, saving transmission time and energy.

For sensors detecting information where a band of frequencies ismeasured, such as measurements of a vibrating source with anaccelerometer, a fast Fourier transform can be locally calculated andonly the frequencies of vibration and the magnitude of vibration need betransmitted, rather than the entire waveform, to reduce the amount ofinformation transmitted and to save energy.

The present inventors provided improved designs of each element of theentire measurement system, including the: vibrating/straining structure,piezo harvesting circuit, sensing circuit, microprocessor, on boardmemory, sensors, and RF data transmitter/transceiver to provide a systemthat operated with low power. The present inventors then built aprototype that both improved on the performance of energy harvestingdevices and that reduced the energy consumption of each element of themeasurement system, including the vibrating/straining structure, piezoharvesting circuit, sensing circuit, microprocessor, on board memory,sensors, and RF data transmitter/transceiver, as shown in FIGS. 3 a, 3b, 4 and 5.

A demonstration energy harvesting circuit was built using a PZT fiber asits input, as shown in FIGS. 3 a, 3 b. The PZT device generates avoltage that is rectified by low forward drop diodes. This rectifiedvoltage is used to charge a storage capacitor. The transfer is purely afunction of the short circuit current of the piezoelectric structure,minus the loss of the rectifier stage, the self discharge of the storagecapacitor, and any leakage current in the switch in its ‘off’ state. Thebehaviour of this configuration is similar to charging a capacitor froma constant current source. The time required to charge the capacitor isinversely proportional to the amplitude of the strain or vibrationapplied to the PZT element at a given frequency of strain, and alsoproportional to the frequency of strain at a given amplitude. Once thevoltage sensing switch detects that enough charge is stored on thecapacitor, the load is connected to the storage capacitor. The load inthis demonstration circuit is a MicroStrain Strainlink RF sensormicrotransmitter. StrainLink is a multichannel, digital wirelesstransmitter system which allows direct sensor inputs from five pseudodifferential (single ended) or three true differential channels.StrainLink features on-board memory, with user programmable digitalfilter, gain, and sample rates and with built-in error checking of pulsecode modulated (PCM) data. Once programmed, these settings reside in thetransmitter's non-volatile memory, which will retain data even if poweris removed. The StrainLink transmitter is compatible with numeroussensor types including thermocouples, strain gauges, pressure sensors,magnetic field sensors and many others. The transmitter can transmitfrequency shift keyed (FSK) digital sensor data w/checksum bytes as faras ⅓ mile on just 13 mA of transmit power supply current. Duringtesting, the transmitter operated for approximately 250 mS on the powerstored in the charged capacitor. This was ample time for the StrainLinkto acquire data from a sensor and transmit multiple redundant datapackets containing the sensed data.

Voltage sensing switch 70 b was implemented using a nano-powercomparator with a large amount of hysteresis. Some design difficultiesarise when using an electronic device to perform such switching tasks.Voltage sensitive switch 70 b itself needs to be powered from the sourceit is monitoring. When the available voltage is near zero the state ofswitch 70 b is indeterminate. This can present a problem when thecircuit is initially attempting to charge the capacitor from acompletely discharged state. In the demonstration circuit as built, theswitch defaults to ‘on’ until the supply voltage to its Vcc exceeds0.7V, then it will turn off until the intended turn-on voltage level isreached. The transmitter draws constant current, except when the supplyvoltage is below the transmitter's regulator threshold. In thiscondition the current increases slightly from the normal operatingcurrent of 11 mA to about l5mA at less than 1 volt. Because of this, andthe fact that the switch is ‘on’ below approximately 0.7 volts, asilicon diode with equal to or greater than 0.7 V forward drop was addedfrom the output of the switch to the transmitter power pin. This allowsthe storage capacitor voltage to charge to the point where the switch isactive before the transmitter starts drawing current. The settings forvoltage trip points were adjusted to 6.3V ‘on’ and 2.9V ‘off’.

In practice, the voltage sensing switch is still falsely ‘on’ at supplyvoltages of up to 1 volt, at which point the diode is already conductingpower into the load. Drawing current from the storage capacitor at thislow voltage slows the charging of the capacitor. This creates aproblematic “turn-on” zone where the capacitor is being drained at thesame time it is being charged. This makes it difficult for the system toinitially charge itself enough to begin operating properly. If enoughstrain energy is applied to the PZT element during initial systemstartup, then this turn-on zone is exceeded, and the system worksproperly.

The present inventors recognized that switching the positive rail e.g. a“high-side switch,” inherently requires some supply voltage to bepresent in order to properly turn the load “off.” This is not the casewith a “low-side switch,” or one in which the minus lead is switched toDC ground. FIGS. 3 a, 3 b, 11 a, 11 b illustrate an improvement to theswitch that will eliminate the turn-on zone problem. It employs both theexisting high side switch implemented with power comparator V1, LTC 150,and the addition of a low side switch in the DC return path of the powersource. The low side switch is implemented with an N channel enhancementmode MOSFET, such as first switch Q1, 72 that has a gate turn-onthreshold higher than the minimum operating voltage of the high sideswitch. This combination eliminates the disadvantages of the high sideswitch and the difficulties with implementing the appropriate switchingfunction using only low side switch components.

High side voltage sensing switch V1 may falsely turn on when storagecapacitor 42′ is charged to between 0.7 and 1.0 volts. No current willflow, however, until the supply voltage exceeds the Vgs voltage of thegate of MOSFET Q1, 72. The Vgs voltage is typically greater than 1.5volts even with so-called logic level MOSFETS that are optimized forfull saturation at logic level (5 volt) gate to source voltage. Once thecharge on capacitor 42′ has exceeded Vgs, the MOSFET will allow currentto pass, but by that point, the voltage sensing circuit will havesufficient supply power to function properly. These changes allow energyharvesting circuit 18′, 18″ to efficiently begin charging itself evenwhen it starts from a completely discharged state.

Efficiency of the energy storage element is an important factor inimplementing efficient designs because the energy may need to be storedfor significant time periods before it is used. In the demonstrationenergy harvesting system, an aluminum electrolytic capacitor wasutilized as the storage element. This is not an ideal choice since itsleakage loss is relatively high. In fact, it can be as much as ten timeshigher than that of the voltage sensing switch used to monitor thecapacitor voltage. To minimize this problem, alternative capacitortechnologies, such as tantalum electrolytic and ceramic, can be used.

No matter what capacitor technology is used, charge leakage is likely tobe a limiting factor in applications where long term storage of chargeis necessary. Batteries, can be used for long term energy storage device42, 42′, and have advantage of essentially zero charge leakage (<1%energy loss per year). Thin film batteries, such as those provided byInfinite Power Solutions, Littleton, Colo., offer advantage of beingable to be charged and discharged in excess of 100,000 times. Inaddition, battery chemistry allows for a battery cell to be continuouslycharged when power is available, as supplied by the PZT. The batterycells have high enough peak energy delivery capability (10 mA pulsedpower) to allow for short bursts of RF communications.

Reduced power consumption is inherently beneficial to the performance ofsystems using harvested energy. A significant reduction in powerconsumption can be realized through the use of embedded software inmicrocontroller 48 that controls the power consumed by the sensors,signal conditioning, processing, and transmission components of theenergy harvesting wireless sensing systems (FIG. 5). By adjusting thetime these devices are on, for example, power consumed can be reduced.In addition embedded processor 48 can be programmed to process and storesensed information rather than immediately transmit, and thereby reducethe frequency of data transmission. Finally the power levels used for RFcommunications can be reduced by bringing a receiver closer to thesensor nodes. This can be accomplished by providing multiple receiversfor a sensor network, by bring an operator with a receiver closer, or byproviding a mobile robot that approaches sensors and reads their data,as more fully described in copending application docket number 115-004,incorporated herein by reference.

The most direct strategy to reduce the power consumed by the sensors andsignal conditioners is to use sensors that do not require power, such asthermocouples, piezoelectric strain gauges, and piezoelectricaccelerometers. For thermocouples, cold junction compensation can beprovided with a micropower solid state temperature sensor (NationalSemiconductor, Santa Clara, Calif.) that typically consumes 20 microampscurrent at 3 volts DC, for a continuous power consumption of only 0.06milliwatts.

A second strategy is to employ sensors that do not need to transmit datafrequently, such as temperature and humidity sensors. There are severalvery low power humidity sensors, for example from Honeywell that can beemployed along with thermocouples or solid state temperature sensors toprovide periodic data updates. For these types of measurements, thereading changes slowly, so energy can be conserved by transmitting thedata infrequently.

A third strategy to minimize the power consumed by sensors 52 is topulse the power to sensors 52 and to time the reading of data from A/Dconverter 50 appropriately. With the sensor on only for a brief periodof time to achieve a stable reading and to obtain that reading forstorage or transmission, much energy can be saved. Microstrain hassuccessfully used this technique for powering and gathering data fromstrain gauges used in medical implants. The current, and therefore thepower, savings that can be realized are significant. For example, a 350ohm strain gauge bridge excited with 3 volts DC will consumeapproximately 8.6 milliamps. If powered continually, this represents apower drain of 25 milliwatts. By only providing the excitation voltageat periodic intervals and performing digital data conversion undermicroprocessor control, we have been able to reduce the sensorexcitation time to 200 microseconds. For applications where a straingauge reading is required every 100 milliseconds (10 Hz), the effectivepower drain is reduced by a factor of 500, (from 25 to only 0.05milliwatts).

Power reductions in the signal conditioning are also realized by usinghighly integrated circuits (IC), such as the AD7714 by Analog Devices(Norwood, Mass.), that use very low power and combine a programmablegain instrumentation amplifier (110 dB CMRR), multiplexer, and 22 bitsigma-delta analog to digital converter. The current consumed by theAD7714 is 200 microamps at 3 volts DC, or 0.6 milliwatts. The AD7714 canbe programmed to accept 3 full differential or five single ended sensorinputs. We have successfully employed this IC for use with foil andpiezoresistive strain gauges, thermocouples, temperature sensors, torquesensors, and load cells. With a rectifier, a differential amplifier andperiodic excitation we have successfully used these ICs with inductivedisplacement sensors.

The power consumed by the embedded processor can be reduced by using lowpower embedded microcontrollers, such as the PIC 16 series fromMicroChip Technologies (Chandler, Ariz.). Such embedded processorsinclude integrated instrumentation amplifiers to facilitate sensorconditioning, and integrated radio frequency (RF) oscillators tofacilitate wireless communications. By including more capability on theprocessor, component count and system complexity are reduced, and thereis a reduction in power consumed. Further reductions in powerconsumption are realized by placing the processor in “sleep mode” whilethe energy harvesting circuit is storing energy in its capacitor bank orbattery. The processor (and its integrated amplifier/RF stage) does notcome out of sleep mode until the energy harvesting circuit detects thatthe stored charge is adequate for the programmed task, such as reading asensor. This prevents the measurement system and processor from loadingthe energy harvesting circuit and allows storage of energy to proceedmost efficiently.

Further reductions in power consumption may be realized by using lowerclock rates for the embedded processor. For example our existingStrainlink digital wireless sensor transmitter product is normally runat a clock rate of 4 MHz, and it consumes 600 microamps at 3 volts DC(1.8 milliwatts). For temperature measurement applications (or any otherapplication requiring relatively infrequent data samples), we can reducethe processor's clock rate to 100 KHz, allowing a more than 20 foldpower reduction to 28 microamps at 3 volts DC (0.084 milliwatts). Formany health monitoring applications, we can improve performance bysimply slowing down the system clock.

The energy required to power sensors, acquire data, and process/storethese data is much lower than the energy required to wirelessly transmitthese data. In the preceding discussion, we have shown thatthermocouples (0 milliwatts) with cold junction compensation (0.06milliwatts) could be combined with a smart microcontroller (0.084milliwatts) and a very low power, highly integrated signal conditioner(0.6 milliwatts) to provide continuous thermocouple readings with apower drain of 0.744 milliwatts. This is in sharp contrast to the RFcommunications section of the electronics, which may require over 10milliamps at 3 volts DC for a power drain of 30 milliwatts in order toprovide adequate wireless range and good margin in electrically noisyenvironments.

By programming the processor to acquire and log sensed data and tocompare these data to programmable threshold levels the frequency of RFtransmission can be reduced to save power. If the sensed data exceeds orfalls below the acceptable operating temperature ranges, then theprocessor transmits its data, along with its address byte. Arandomization timer is be used to insure that if multiple transmittersare transmitting their data (or alarm status) simultaneously, theprobability of RF collisions is statistically small, as described in apaper entitled “Scalable Wireless Web Sensor Networks,” SPIE SmartStructures and Materials, March, 2002, by Townsend et al. In the eventthat threshold levels are not crossed, stored summary data, such asmean, maximum, minimum, and standard deviation, are periodicallytransmitted over time intervals, such as hourly or daily. Transmissionof processed data, such as these trends, and periodic transmission ofthis data saves more energy.

The processors may also be programmed to acquire bursts of data from avibrating structure using an accelerometer. These data may be processedusing average fast Fourier transform (FFT) and power spectral density(PSD) analyses. The processed data would allow the RF link to transmitonly the fundamental vibration frequencies, which would greatly reducethe amount of data that is transmitted and thereby greatly reduce the“on-time of the RF link.

To further reduce power consumed by the energy harvesting sensing nodes,we could reduce the RF communications power levels at the expense ofrange. For some applications, it may be possible for Navy maintenancepersonnel to approach an area where shipboard monitoring nodes have beenplaced. The energy harvesting monitoring nodes may also include RFtransceivers, which would provide for bi-directional communications.Instead of only periodically transmitting sensed data, these nodes areprogrammed to periodically activate their integral receiver to detectthe presence of maintenance personnel over the wireless link. A handheldtransceiver carried by maintenance workers would query various nodes onthe network and collect their data into the handheld device. This wouldgreatly reduce the need for long range wireless data communications, andtherefore would allow for reduced RF power levels at the remote energyharvesting sensor nodes. Microstrain has developed a high speed datalogging transceiver product that could be adapted to this purpose.

The vibrational energy harvesting unit is illustrated schematically inFIG. 1 a-1 d. It consists of circuit board 25 that is rigidly fixed tosome vibrating component, such as vibrating machine 19. Leaf spring 22is mounted to this base in a cantilever configuration. Proof mass 24 issuspended at the free end of the leaf spring, and this can be adjustedto more nearly provide a resonance frequency close the vibrationfrequency. One or more PZT elements 20 are bonded to the surfaces ofleaf-spring 22 such that when spring 22 deflects, PZT 20 will undergotensile/compressive strains and therefore be stimulated to generate anelectrical output suitable for input into energy harvesting circuit 18′,18″. To maximize the output of PZT 20, leaf spring 22 is preferablyconstructed using a “constant strain” profile, as shown in FIG. 1 c,such that the strains experienced by the PZT elements are uniform alongtheir length. To provide a constant strain profile, leaf spring flexureelement 22 can have a taper, as shown in FIG. 1 c. Enclosure 26surrounds the device to keep contaminants out, and to make the deviceconvenient to handle and damage resistant.

Enclosure 26 measures approximately 5×50×150 mm and leaf spring flexureelement 22 has adjustable proof mass 24 of between 100 and 500 grams.Tuning the unit will be accomplished by adjusting the size of proof mass24, which can be washers, as shown in FIG. 1 c. The energy harvester iscapable of generating sufficient energy to intermittently power atransmitter and several low power sensors, as shown in FIGS. 3 a, 3 b,11 a, 11 b.

As described herein above, a rotating shaft can also be a source ofenergy for harvesting. In fact, one of the most important applicationsfor wireless sensors is the measurement of torque on rotating shafts.Ironically, this application has been neglected by researchers workingin the area of energy harvesting. Commercially available torquemeasurement systems for use on rotating shafts currently use slip rings,battery powered telemetry, or external alternating current (AC) magneticfield powering. But slip rings are costly and unreliable. Batterieseventually die and must be replaced. External AC powering systems mustbe powered themselves, and require close physical proximity to operatereliably. External powering systems can also add significant cost,complexity, and size. The limitations of existing systems have preventedwide acceptance of shaft monitoring systems, but there are literallyhundreds of millions of rotating shafts on cars, trucks, machines,pumps, etc. that could benefit from being monitored.

The present inventors recognized that rotating shafts may not alwaysspin at constant rates, and may even come to a stop, therefore theamount of energy supplied by the rotating shaft may vary substantially.Thus, they recognized that the energy harvesting system should becapable of storing energy harvested from the rotation over time. Thisstored energy insures that data may be delivered even when the shaft isrotating slowly or there is otherwise insufficient ambient mechanicalenergy to power the sensing and transmission system continually.

The present inventors also recognized that the advent of tiny, low cost,battery-less digital wireless communications, especially when combinedwith smart sensors and internet data delivery, would allow extremelyefficient condition based maintenance (CBM) of these rotating shaftsystems. They recognized that providing energy storage and data storagewould allow data collection and communications when the shaft is notrotating or when insufficient energy has been stored to allow immediatetransmission. Energy storage can be provided by a capacitor, arechargeable battery, or another energy storage device. They recognizedthat the improvements provided herein would allow defects inlubrication, bearings, etc. to be recognized before failure occurred,allowing considerable savings to be realized through prevention ofequipment downtime caused by sudden equipment failures.

The present inventors created a new class of energy harvesting wirelesssensor systems which they termed “Spin Powered”, and which they designedfor use on rotating shafts. Energy generated by the motion of thespinning shaft is harvested, and this energy is stored over time. Onceenough power is available or enough energy has been stored to power thetransmitter, the system will transmit data collected by its sensors toprovide a direct measure of shaft torque, rate of rotation, andmechanical power driving the shaft. A microprocessor and a micro-powerdigital RF link to a remote receiver may be used for controlling thetransmission.

Shaft mounted energy harvesting system 115 includes one or more coils116 deployed on rotating shaft 117, as shown in FIG. 13. One or morestationary permanent magnets 118 a are placed on fixed supports adjacentrotating shaft 117. Preferably permanent magnet 118 a is fabricated of amaterial such as AlNiCo, neodymium iron boron or another magneticmaterial. As shaft 117 spins, coils 116 mounted on shaft 117 spin aswell. As each coil moves through the field produced by permanent magnet118 a that coil experiences a changing magnetic field. The changingfield experienced by each coil induces an emf or an electrical pulse inthat coil which can be converted to DC power using rectifiers 119, asshown in FIG. 13 and in more detail in FIG. 14. Spinning coils 116 onshaft 117, are shown in schematic fashion as inductors (L) in FIG. 14.Rectifiers 119, such as semiconductor diodes or Schottky barrier diodes,are used to rectify the current pulses induced in coils 116.Alternatively, a full wave rectifier could be used, such as rectifier40, shown in FIG. 11 a.

Thus, pulses of electricity are generated in coils 116 mounted on shaft117 from the motion of shaft 117 and coils 116 through a stationarymagnetic field, and this energy can then be used to power componentsmounted on rotating shaft 117 without any electrical connection to shaft117. In addition to generating energy from magnetic induction the pulsesof electricity generated in one of the coils 116′can be used formeasuring angular speed and rotations per minute, or RPM, as furtherdescribed herein below. Other than coils, devices such as Weigand wireelements, could be used to generate the emf in place of coils 116.

Stationary permanent magnet 118 a can be a horseshoe magnet whichconcentrates magnetic flux lines through spinning coils 116, as shown inFIG. 14. Flux concentrating pole pieces with a straight permanent magnetcould also be used. This concentration of flux enhances the efficiencyof shaft mounted power generating and energy harvesting system 115. Inorder to further enhance the amount of power generated, additionalstationary permanent magnets 118 a could be used or more coils 116 couldbe arranged around the periphery of spinning shaft 117, as shown in FIG.15. It is also possible to substitute stationary electromagnet 118 b forpermanent magnet 118 a, as shown in FIG. 13. However, stationaryelectromagnet 118 b would require a power source, whereas stationarypermanent magnet 118 a does not require a connection to a power sourcein order to provide a DC magnetic field. Magnetic coupling betweenmagnet 118 a and coils 116 could be improved by reducing the gap therebetween and by making the lateral dimensions of coil 116 more closelymatch the lateral dimensions of pole pieces of stationary permanentmagnet 118 a. Also polishing the faces of pole pieces 119 usually allowsbetter magnetic coupling.

The present inventors improved energy harvesting circuitry for use withsuch a shaft mounted induction powering system. They mountedmicrocontroller or microprocessor 120, non-volatile memory 122, such asEEPROM, data conversion elements, such as A/D converter 124, sensorpower supply 126, analog circuitry for analog sensor signal conditioning128, and sensor signal conditioning power supply 130 on printed circuitboard 132, as shown in FIG. 13.

They also provided connection to one diode 119′ which is connected toinput 136 a to sensor signal conditioning circuitry 128 for detectingthe angular rate of rotation of spinning shaft 117 on printed circuitboard. Sensor signal conditioning circuitry 128 uses the current pulsesgenerated by coil 116′ as rectified in diode 119′ to determine angularrate of rotation ω based on the time elapsed between pulses generated inthis coil by revolution of shaft 117. A crystal (not shown) may beprovided for use with microcontroller 120 for providing accurate time tomicrocontroller 120 for providing the time reference for performing thiscalculation. Alternatively, an on-board RC oscillator (not shown) can beused in microcontroller 120 to provide time. A crystal is generally moreaccurate since an RC oscillator may drift with time and temperature.

The signal at diode 119′ occurs once per revolution of shaft 117 as coil116′ to which diode 119′ is connected passes permanent magnet 118 a, asshown in FIGS. 13 and 14. Rate of rotation detector 133 includes diodes134 and resistors 135 which are configured to convert pulses from diode119′ into logic levels at input 136 a that microprocessor 120 caninclude with its internal time measurement to calculate rpm.

Printed circuit board 132 also includes radio power supply 140 and RFtransmitter or transceiver 142, both connected to microprocessor 120, asshown in FIG. 13. RF transmitter or transceiver 142 transmits data itreceives from microprocessor 120 through antenna 144. Base station 146receives transmissions from RF transmitter or transceiver 142 throughantenna 148 and can display data on display 150 or transmit data toother receivers through the internet represented by cloud 152, as alsoshown in FIG. 13.

Printed circuit board 132 also includes additional electronic componentsused to facilitate harvesting energy from the electrical pulses providedby rotating shaft 117 and coils 116 moving in the stationary magneticfield of stationary permanent magnet 118 a. Rectifiers 119 receivingsignals from each coil 116 are ganged together, and their output may goto battery charging circuit 158 and energy storage device 160 torecharge energy storage device 160, as shown in FIG. 13.

Energy storage device 160 may be a very low leakage, high capacitycapacitor, or “super capacitor,” such as those provided by thePanasonic's Gold Gap series, Matsushita, Corp. Japan. Alternatively,rechargeable electrochemical batteries capable of a very high number ofrecharging cycles (Infinite Power Solutions, Golden, Co) may bedeployed. Energy storage device 160 may be a single or multi-celledlithium-ion battery or an electrochemical battery. Lithium-ion chargecontroller, such as the BQ2400X available from Texas Instruments,properly charges a lithium-ion battery and protects the battery fromsuch improper input as overcharging, and overheating. Rechargeablebatteries provide a reliable long term storage means, which isespecially important in those applications where system start-up torquesneed to be recorded or transmitted since during startup shaft 117 maynot be providing enough energy for recording or transmission.

Battery charging circuit 158 can optionally include a nano-ampcomparator switching circuit, similar to that provided in thepiezoelectric energy harvesting system illustrated in FIG. 11 b. Anano-amp voltage comparator monitors output voltage of rectifier 119 toensure that adequate charge is available to store on charge storagedevice 160. If the nano-amp comparator determines that a high enoughvoltage is being generated then it switches that voltage into energystorage device 160.

The charge level provided on energy storage device 160 provides anunregulated output level which is then regulated to a voltage level Vccin regulator 162, as shown in FIGS. 13 and 14. This regulated Vcc isprovided to the various power supplies on PC board 132, including apower supply for microprocessor 120 and power supplies 126, 130, 140 forsensors, signal conditioning, and RF transmission. Regulator 162, mayinclude an LP series regulator from National Semiconductor Corporation,Santa Clara Calif., part number LP2980IM5-3.0.

Battery charging circuit 158 insures that the battery can be tricklecharged when low energy is being harvested or charged more rapidly inthe event that higher levels of energy are available. As there are manytypes of batteries available, an appropriate commercially availablecharge controller specific to the battery chosen is desirable to controlthe charge process.

For a spinning shaft 117 generating a large amount of electrical energya small energy storage device 160, such as a capacitor or a smallrechargeable battery, is adequate because it can be kept fully chargedeven with all devices continually powered by processor 120. However, fora spinning shaft 117 that is spinning slowly or intermittently, or isotherwise generating little electrical energy, preferably a largerbattery or a larger capacitor is used to store energy and intermittentlyrun power consuming electronics, such as microprocessor 120 and thesensing, data logging, and transmission elements shown in FIG. 13.

Sensors connected to inputs 136 a, 136 b, 136 c include rate of rotationsensors strain gauges, temperature sensors, and other sensors, such asaccelerometers, and these sensors may be located on printed circuitboard 132 or they may be directly mounted on shaft 117, as shown in FIG.15, and connected to PC board 132 with wires 163.

The present inventors found that they could epoxy torsional straingauges 164 a, 164 b to shaft 117, as shown in FIG. 15, for measuringtorsional strain in shaft 117. They could then convert this measuredstrain to torque of shaft 117 using the equation below, where T is thecalculated torque, ε is the torsional strain as measured by torsionalstrain gauge 136 b, Do is the outer diameter of shaft 117, Di is theinner diameter of shaft 117, E is the modulus of elasticity of thematerial of which shaft 117 is fabricated, and v is the Poisson RatioPoisson ratio is material property having to do with width v. stretch ofthat material.

$\begin{matrix}{{{Torque}(T)} = \frac{ɛ*{\pi ( {{Do}^{4} - {Di}^{4}} )}E}{16{D_{o}( {1 + \nu} )}}} & (1)\end{matrix}$

The present inventors executed the calculation of equation (1) withfirmware embedded in microcontroller 120 on printed circuit board 132that could perform the arithmetic functions of equation (1), and theywere able to compute the torque for shaft 117 from pulses provided byset of coils 116 mounted on shaft 117.

Using the same calculating firmware they were also able to compute powerprovided by spinning shaft 117, which was calculated by multiplying thecalculated torque T by the angular rate ω derived from the pulsesprovided by coil 11640 , diode 119′, and rate of rotation detector 135.

P=wT  (2)

where P is the mechanical power provided by shaft 117 and ω is theangular rate of rotation expressed in radians/sec.

Note that the angular rate is actually measured directly from pulsesprovided by coil 116′ and diode 119′. Thus coil 116′ and stationarypermanent magnets 118 a are used both for power generation and for RPMor angular rate sensing to compute mechanical power of shaft 117. Theability to monitor RPM, with coil 116′ and diode 119′, (or with Wiegandeffect devices), along with the ability to monitor torque of shaft 117from torsional strain in shaft 117, as described herein above (or withcommercially available torque sensors connected to input 136 b) providesthe needed data for the microprocessor 120 to automatically compute theinstantaneous mechanical power produced by the system that is drivingthe shaft.

In the preferred embodiment, full torsional strain gauge bridge 164 a,164 b is deployed (45 degree chevron patterns) is bonded on oppositesides of the shaft, as shown in FIG. 15, to provide a full bridge withmaximum sensitivity to torsional strain while cancelling bending andaxial strains. It is well known how to install a chevron pattern on ashaft to cancel bending and axial strains.

Preferably coils 116 and printed circuit board 132 that encompasssensing package 166 are mounted on split cylindrical ring 168, as shownin FIG. 15 so shaft 117 does not need to be removed or altered forinstallation of sensing package 158. Split ring 168 includes splitclamps (not shown) to attach split cylindrical ring 168 on shaft 117.Strain sensing elements 164 a, 164 b are preferably glued to shaft 117,and wiring 170 from strain sensing elements 164 a, 164 b are connectedto printed circuit board 132.

Alternatively a commercially available torque sensing device (not shown)can be clamped to shaft 117 or a torque transducer (not shown) can beput in line with shaft 117, but this requires interrupting shaft 117.The present system for harvesting energy from shaft rotation can also beused for powering devices such as a torque sensing device or a torquetransducer, commercially available from Micromeasurements, Inc.,Raleigh, N.C.

Torsional strain sample rate can be in the range from 0-1000 Hz istypical and it can range to 40 kHz or higher. Torsional strain sampleduration of 25 microseconds is typical, depending on the speed of A/Dconversion, leaving a significant amount of time between samples takenat 1000 Hz or lower. During this time between samples the presentinventors found that they could program microprocessor 120 to go intosleep mode and to turn power to other components off, substantiallyreducing the amount of power consumed by system 115. They also foundthat they could program sample rate in microprocessor 120 and that theycould program microprocessor to automatically adjust sample ratedepending on available stored energy in energy storage device 160 andthe rate energy is being accumulated or depleted on that device.

RF transmitter portion of transceiver 142 may operate at 916 MHz,narrowband FSK, and may have a software programmable carrier frequencyand RF power level. In other countries other appropriate frequencies canbe used. Alternatively direct sequence spread spectrum can be usedrunning at 2.4 Ghz, an emerging standard for low power wireless sensorsreferred to as zigbee. Alternatively, for periodic transmission 418 MHZcan be used in the US.

RF receiver portion of transceiver 142 may operate at 400-930 MHz,narrowband FSK, and have serial RS-232, USB, and high level 0-5 voltsanalog outputs. Other possibilities, as described for the RF transmittercan also be used.

Sensors for measuring other parameters of shaft 117, such as bendingstrains, or axial tensile/compressive strains can also be included. Athree channel version of SG-Link can be used with judicious placement ofstrain gauges to make these other measurements. Rechargeable-batterypowered wireless strain gauge systems, commercially available under thetrade names V-Link by MicroStrain, Inc. (Williston, Vt., USA), couldalso be used. V-Link has four differential sensing channels, atemperature sensor, and 3 additional analog inputs, for a total of 8sensing channels.

Other sensors can also be used, such as accelerometer 136 d, to measureother parameters, such as vibration of a shaft that could be produced byout of balance, bearing failure, or suspension failure since problems indevices connected to the shaft could be transmitted back to the shaftand picked up by accelerometers or other sensors. Since accelerationincreases with angular rate accelerometer 136 d can also be used todetect angular rate and rpm without need for the magnet, coil, anddiode. Correction for gravity, if needed, can be included in firmware.Alternatively, accelerometer 136 d can give additional information aboutthe health of bearings from vibration it measures.

Data from diode 119′ and strain sensor 154 b connected to inputs 136 a,136 b are conditioned in signal conditioning 128 and then received byA/D converter 124 and microprocessor 120, as shown in FIG. 13.

Sensor signal conditioning 128 includes sensor amplification, automaticshunt calibration, hardware/software programmable sensor offsetadjustment, and hardware/software programmable sensor gains, asdescribed herein above and in commonly assigned U.S. Pat. No. 6,529,127and in the 115-005 application, both of which are incorporated herein byreference, and in user manuals available from MicroStrain, Inc. Bridgecompletion resistors may also be included in signal conditioning 128 forthose applications where the sensor does not use a full Wheatstonebridge, as also shown in these user manuals, to provide a fullWheatstone bridge to the signal conditioning. Sensor amplification,automatic shunt calibration, hardware/software programmable sensoroffset adjustment, and hardware/software programmable sensor gains arecurrently available in rechargeable-battery powered wireless straingauge systems, commercially available under the trade names V-Link andSG-Link by MicroStrain, Inc. (Williston, Vt., USA). Devices such asV-Link or SG-Link can be used along with the energy harvesting powersource provided herein to provide these features.

For strain sensing applications, the ability to wirelessly programsensor offsets and gains has been an important feature of the signalconditioning, because strain gauges typically exhibit significant offsetdue to changes in resistance induced during installation. Furthermore,gain programmability is important because in many applications the fullscale strain output is not known when the device is manufactured, andtherefore the system gain may need to be adjusted after installation.

The ability to monitor temperature simultaneously with strain allowssystem 115 to automatically compensate and correct for changingtemperature on the offset and gains associated with the torque. A lookup table or mathematical relationship describing how the offset andgains change with temperature determined in a calibration step is usedto automatically make adjustments as temperature changes. Thecalibration step and generation of the look up table or mathematicalrelationship may be done in the factory. Alternatively, gain can beadjusted in the field with a shunt calibration, as shown in FIG. 16.Microprocessor controlled switch 180 is closed to switch knownresistance 182 in parallel with one arm 184 of Wheatstone bridge 186that includes strain sensing elements 164 a, 164 b. Switching in knownresistance 182 provides a known resistance shift into arm 184 ofWheatstone bridge 186, which simulates a known sensor signal into bridge186, and gain calibration can thereby be achieved by adjusting digitaltrimpot 188 to provide a desired full scale output when the shunt isapplied.

Offset associated with the sensor signal conditioning 128 can also beautomatically removed in the field by shorting inputs to instrumentationamplifier 190 with microprocessor controlled switch 192, as also shownin FIG. 16, to remove all strain sensing elements, such as Wheatstonebridge 186, from sensor signal conditioning circuit 128. Shortingmicroprocessor controlled switch 192 provides equal input signals toboth inputs of instrumentation amplifier 190. Any non-zero output is theoffset. Microprocessor 120 can then automatically adjust the offsetvoltage using offset programable digital potentiometer 192 to re-zerothe offset so that the output of amplifier 194 as measured through A/Dconverter 124 is now zero. An offset is provided to programable offsetdigital trimpot 192 to make this output zero. With equal amplitudes atboth inputs of amplifier 194, its output should now be zero.

RF chokes 194 a, 194 b are provided at both inputs to instrumentationamplifier 190 to eliminate high frequency noise. Output of signalconditioner 128 goes to A/D converter 124 and then to microprocessor120.

Management of power is particularly important for shafts that may spinslowly and generate less energy, for small diameter shafts or wherespace around the shaft is small so multiple stationary permanent magnets118 a cannot be deployed, or for systems that otherwise have to beminiaturized. As mentioned herein above, the present inventors improvedefficiency of energy use by adjusting sampling rate for energyavailable, sleeping processor 120 between samplings, and shutting downpower supplies 126, 130, 140 for sensors, sensor signal condition and RFtransmission between samplings. In addition they provided for datastorage on non-volatile memory 122 and provided for data reduction inmicroprocessor 120, thereby reducing the amount of transmission andduration of transmission required.

Sensors connected to sensor inputs 136 a, 136 b, 136 c are connected tosensor signal conditioning circuit 128 which is powered by sensor signalconditioning power supply 130. For strain and torque sensing elements,sensor signal conditioning power supply 130 are preferably very lowpower voltage regulators, such as LP series regulators available fromNational Semiconductor Corporation, Santa Clara, Calif. Preferably powersupply 130 is a voltage regulator with an enable pin and control line130′ from microprocessor 120 enables microprocessor 120 to provide asignal to turn on or turn off the voltage regulator. Preferably a numberof other such control line regulators, such as power supplies 126 and140 are provided on printed circuit board 132, as shown in FIG. 13.Control lines 126′, 140′ provided from microprocessor 120 controloperation of these power supplies, such as sensor power supply 126 andradio communications power supply 140. This separate control over eachpower supply 126, 130, 140, allows microprocessor 120 to only turn onthose power supplies and those elements that are needed for a requiredfunction, such as transmission, while leaving other power supplies andother elements off, therefore minimizing the instantaneous and averagepower consumed by system 115. This separate control is particularlyuseful when the amount of energy that can be harvested from spinningshaft 117 is low due to slow rotation of shaft 117. For example separatecontrol over each of these power supplies allows microprocessor 120 toselectively turn on selected portions of the circuit to meet the needsof a given application without turning on other portions. Alternatively,separate control over the various power supplies also allows powerconsumption of shaft mounted energy harvesting system 115 to bedetermined based on the available energy from the spinning shaft energyharvesting portion of the system. For example sample rate can beadjusted by microprocessor 120 by adjusting time between signalsenabling power supplies 126 and 130 proportional to RPM of shaft 117, asone example of such control.

Alternatively to controlling devices on spinning shaft 117 based on RPMof shaft 117, microprocessor 120 can also base control on charge levelof energy storage device 160 or based on rate of depletion of chargefrom energy storage device 160. Rechargeable energy storage device 160can be queried by microprocessor 120 with a voltage divider or a batterygas gauge indicator, as is well known in the art. By measuring chargestate and then later measuring charge state again one can also determineif energy storage device 160 is depleting or charging and one can alsodetermine the rate of depletion or charging. Based on this informationprocessor 120 can adjust such things as the rate of sampling sensors,the length of time they are sampling each time, and how often totransmit data. Microprocessor 120 can be remotely programmed to changethese parameters or it can automatically adjust itself, as shown in theflow chart in FIG. 17, depending on the state of charge and the rate ofdepletion of charge as well as depending on the requirements of themeasurement application.

In the first step, microprocessor 120 determines whether it is time towake itself up, as shown in step 200. If it is not time, microprocessorremains in sleep mode. Alternatively, microprocessor may awaken uponreceiving a signal from a base station. In either case, upon awakening,microprocessor 120 assess charge on energy storage device 160, as shownin step 201, based on open circuit voltage or voltage across a voltagedivider or another battery charge circuit. Alternatively it may use thespeed of rotation of shaft 117. Based on this assessment, microprocessordecides whether charge on energy storage device 160 is sufficient, asshown in step 202. If not, microprocessor 120 goes back to sleep andawaits additional time or another signal. If charge is sufficient,microprocessor 120 sends a signal to power up sensor electronics, suchas strain sensor 164 a, 164 b, as shown in steps 203 and 204.Microprocessor includes a delay time of about 50 microseconds for it tofully stabilize after awakening from sleep mode, as shown in step 205and then microprocessor sends a signal to the power supply that powers asensor, as shown in step 206 which initiates sampling. In the next step,microprocessor turns off the signal to the power supply powering thesensor, as shown in step 207, sends signals powering down sensorelectronics, as shown in step 208, and puts itself to sleep as shown instep 209 to await the next time to wake up.

Microprocessor 120 has control line 140′ to RF communications powersupply 140 and communication lines 142′ to RF transmitter or transceiver142 for transmitting data externally. Microprocessor 120 can therebyadjust the frequency of external transmission by adjusting time betweensignals enabling power supply 140 based on energy available. Similarly,microprocessor 120 may also have both control and communication lines(not shown) to nonvolatile memory 122 for data collection and dataprocessing. To further reduce the consumption rate of powermicroprocessor 120 can similarly control other power using elements ofsystem 115 by enabling and disabling power supplies to those elements,as described herein above. For example such control lines may be used tocontrol the state of sensors connected to inputs 136 a, 136 b, 136 c topermit minimizing the energy consumed by these sensors along withcontrolling sensor power supply 126 and controlling sensor signalconditioning power supply 130 to which they are connected.

With control over the various power supplies and sensors, during a timeperiod when data from sensors is being sampled, microcontroller 120 cansubstantially reduce power consumption by turning power off to suchdevices as strain gauge 154 b, strain gauge power supply 126, straingauge signal conditioning power supply 130 and thereby to signalconditioning 128. In addition, between turn-on signals provided bymicroprocessor 120, microprocessor 120 goes into sleep mode. With allthese devices powered off between signals from microprocessor 120, powerconsumption can be reduced by orders of magnitude. For example power tosensor, signal conditioner, and microprocessor may be off for 10 ms andon for 0.25 ms. Thus power is on only 2.5% of the time.

In sleep mode microprocessor 120 is not entirely turned off. At leastthe timer function of microprocessor 120 is still enabled, and the timeis used in microprocessor 120 to determine when microprocessor 120 wakesup from sleep mode to provide turn-on signals to other components and toprovide other scheduled operations.

In order to best take advantage of these energy savings, built-infirmware in microcontroller 120 was programmed to wake up andautomatically send turn-on signals to the sensors, sensing portions ofthe electronics, and the RF power supplies, 126, 130, 140, whilesynchronously performing analog to digital conversions of data receivedin A/D converter 124 and providing RF communications with transmitter142.

In another embodiment of the power saving scheme, microprocessor 120 isprogrammed to wake up and automatically send turn-on signals to sensorsconnected to inputs 136 a, 136 b, 136 c, power supply 130 for sensorsignal conditioning 128, a/d converter, and non-volatile memory 122, butRF transmitter or transceiver power supply 140 is left off. In thisembodiment data is transmitted out at a much lower rate than data issampled and stored. Microprocessor 120 provides data it receives tononvolatile memory 122 during the turn-on signal time for latertransmission. Microprocessor goes back into sleep mode between samplingsuntil time for the next turn-on signal arrives. This embodiment issimilar to one herein above for the piezoelectric energy harvestingsystem, in which data was not transmitted in real time. Substantialenergy savings are achieved by storing data in non-volatile memory 122and then transmitted later, when charge in energy storage device 160reaches a pre-determined level.

A circuit similar to that used in the piezoelectric energy harvestingdesign described herein above and in FIG. 3 a can be used to storeenergy and trigger transmission of data when sufficient energy has beenstored in energy storage device 42, as shown in FIG. 18. Energy storagedevice 42 is preferably a low leakage capacitor, such as a 47 uFcapacitor. A larger capacitor can be used but it may take longer tocharge depending on the rotation speed of shaft 117, the number of coils116, and the number of permanent magnets 118 a. Alternatively energystorage device 42 could be a rechargeable battery.

With microprocessor 120 in sleep mode between turn-on signals, and otherpower using devices turned off between turn-on signals, this embodimentprovides for very efficient use of power harvested from potentiallyslowly spinning shaft 117.

To illustrate this point, we compared the power required for fivedistinct modes of operation: (1) data processing and RF transmission ofprocessed data: 45 milliwatts; (2) processing/logging of sensed datawithout transmission: 5.0 milliwatts; (3) pulsed power at 10 Hz andtransmitting at this 10 Hz rate but with microprocessor 120 in sleepmode and all other elements turned off between turn-on signals: 0.75milliwatts; (4) pulsed power at 10 Hz and not transmitting but storingdata in a non-volatile memory, and with microprocessor 120 in sleep modeand all other elements turned off between turn-on signals: 0.300milliwatts; and (5) sleep mode and all other elements turned off: 0.02milliwatts.

Furthermore, by collecting and storing data from strain sensor 154 b innon-volatile memory 122 during one or more revolutions of shaft 117, andthen analysing the data in microprocessor 120 so as to reduce the amountof data to be transmitted, and then transmitting only the result of theanalysis with RF transmitter or transceiver 142, time for transmissionmay be reduced and power for transmitting may be further conserved, ascompared with transmitting all the raw data streaming from sensorsconnected to inputs 136 a, 136 b, 136 c. Such a scheme is described inthe papers, “Wireless Strain Measurement Systems, Applications &Solutions,” by Arms et al., Proceedings of the Joint Meeting of theNational Science Foundation and the European Science Foundation forstructural health monitoring, Strasbourg, France, October 2003,incorporated herein by reference, and available from MicroStrain, Inc.,and “Harvesting Strain Energy for Wireless Sensor Networks,” by S. Armsand C. Townsend, Proceedings of the First International Workshop onAdvanced Smart Materials and Smart Structures Technology, Honolulu, Hi.,Jan. 12-14, 2004. By periodically transmitting information only afterdata analysis in microprocessor 120, such as the maximum, minimum, andaverage of the torque, RPM, and/or power from the shaft, significantpower savings may be realized as compared with transmitting raw data.

If shaft 117 is spinning at a very low angular rate or low RPM,microprocessor 120 may need to wait until sufficient energy has beenstored in energy storage device 160, as more fully described hereinbelow, before sending turn-on signals for collecting and storing thedata in non-volatile memory 122. Microprocessor 120 can assess the levelof charge on energy storage device 160 by connecting energy storagedevice 160 to voltage divider 164 consisting of two high resistances.Voltage divider 164 reduces the voltage of energy storage device 160 toa voltage that microprocessor 120 can read directly with one of its onboard 8 bit A/D converters 124 and also provides a load for moreaccurate assessment of the state of charge. From measuring this voltageat different times microprocessor 120 can also tell whether chargestorage device 160 is charging or discharging. Voltage divider 164 iscontrolled by microprocessor 120 using switch 166 to avoid current drawwhen microprocessor 120 is not monitoring the state of charge of energystorage device 160.

By contrast, when shaft 117 is spinning at a high angular rate or highRPM, microprocessor 120 will preferably be programmed to provide a highrate of turn-on signals to sample the torsional strains on shaft 117 ata higher sampling rate and transmit data frequently. This embodimentallows the system to adapt its sampling rates, sample duration, storagerate, and transmission rate depending on the amount of energy availablefrom shaft 117 in the particular application, and to adapt to thechanging operating conditions of spinning shaft 117, coils 116, andenergy storage device 160.

For very slowly rotating shafts, Wiegand wires (not shown) may bedeployed for energy generation, rather than coils 116. Wiegand wireshave advantage in that their current pulse amplitude is independent ofthe rate of change of magnetic field strength. Conversely, coils 116will produce prodigious energy at higher RPMs, but their ability toproduce energy at low RPM is much less than they produce at higher RPM.

The present inventors demonstrated pulsed operation using a threechannel, 1000 ohm/gauge, wireless strain gauge system. The system, ifcontinuously powered, on average drew about 25 milliamps from a threevolt regulated power supply. By pulsing energy to the electronics &communications link to provide measurements at a rate of 10 measurementseach second for 50 to 100 useconds, the average current drawn wasreduced by a factor of 100, down to about 250 microamps. This low powercapability is enabling for long term battery operation, remote poweringby external fields, and energy harvesting.

As shown in FIG. 19, if network of multiple wireless nodes 220 aredeployed on a single shaft or on multiple shafts 221, 222, 223 or onmultiple machines, pumps, etc. another strategy to save power is toremotely command each wireless sensing node 220 a, 220 b, 220 c frombase station 224. In one embodiment all components on all machines arekept in off state or in sleep mode until this command is received frombase station 224. Alternatively, each node 220 a, 220 b, 220 c canautomatically collect data from its sensors and store its data in itsown non-volatile memory 122 a, 122 b, 122 c in that node but the data isonly transmitted to base station 224 when a signal is received from basestation 224 showing that base station 224 is nearby and ready to receivedata.

Base station 224 can be mobile and located on a robot as described inpatent application Ser. No. 10/379,224, incorporated herein byreference. Alternatively, a person can walk around with base station224. Base station 224 can also be stationary.

If each shaft 221, 222, 223, pump, motor, or other machine is identifiedby a unique identification code associated with just that shaft thenbase station 224 can receive from the various nodes 220 a, 220 b, 220 con network 220 and identify data from each shaft 221, 222, 223 by itsunique identification code.

Shafts 221, 222, 223 and wireless nodes 220 a, 220 b, 220 c monitoringthem can be located on mobile platforms. For example, shaft 221 can bethe cam shaft or drive shaft, or another spinning part on a truck. Basestation 224 can be on the truck too, but off the rotating shaft, and canuse computer 226 to store and analyse data and cell phone 228 to call ahome base to let the home base know that a particular shaft is operatingoutside its specifications and requires service.

In some industrial facilities hundreds of pumps or motors may be runningsimultaneously. This embodiment provides a way to monitor all of them atonce by harvesting energy from the spinning shaft of each pump or motor,monitoring the speed, torque, and power of each spinning shaft withapparatus on each spinning shaft, and transmitting the data from theshaft.

Alternatively, in some cases the energy conserving features describedherein above may save so much energy as to eliminate the need for energyharvesting to recharge a battery. This may be the case if sampling atlow frequency, such as 1 Hz and if, as described herein above, theprocessor is in sleep mode and other electronics are turned off betweensamplings. In such a case some batteries may provide energy lastingseveral years without recharging, and in such a case energy harvestingfor recharging can be avoided. Thus, the energy saving schemes providedherein may satisfy requirements in some cases without the need forenergy harvesting from the spinning shaft.

A paper, “Civil Structure Strain Monitoring with Power-EfficientHigh-Speed Wireless Sensor Networks,” by J. H. Galbreath, C. P.Townsend, S. W. Mundell, M. J. Hamel, B. Esser, D. Huston, S. W. Arms,Proceedings of International Workshop for Structural Health Monitoring,September 2003, Stanford University, Palo Alto, Calif., incorporatedherein by reference, describes addressable, wireless strain sensingnodes that respond to the following base station broadcast addressand/or node specific address commands:

Wake up, listen for commands, log or send data as commanded (or back tosleep)

Wake up, log information when an event or threshold crossing is detected

Wake up, transmit data periodically, go back to sleep

These capabilities are included in the commercially available SG-Linkand G-Link products mentioned herein above that embody the signalconditioning, processing, and turn-on signal capabilities describedherein. However, those nodes do not have microprocessors that go back tosleep between turn-on signals to the other devices. They have on-boardrechargeable batteries for power and do not have provision for energyprovided from energy harvesting.

The addressable sensing nodes described in the paper feature 2 Mbytes ofon-board, non-volatile memory for data storage, 2000samples/second/channel logging rates, 1700 samples/sec/channelover-the-air data rates, bi-direction RF link with remote offset andgain programmability, compact enclosure, integral rechargeable Li-Ionbattery, and on-board temperature sensor. Typical performancespecifications for the wireless strain sensing node combined withconventional piezoelectric foil strain gauges (1000 ohm) are providedbelow:

Temperature coefficient offset 0.007%/deg C. (tested from +20 to +50 degC.)

Temperature coefficient span 0.004%/deg C. (tested from +20 to +50 degC.)

Operating temperatures −20 to +85 deg C.

Programmable full scale range: 1000 to 5000 microstrain

Resolution +/−2.5 microstrain (tested w/anti-aliasing filter bandwidth0-500 Hz)

Preferably base station 146 has analog output 240 to provide feedback tomotor controller 242 to adjust operation of electric motor or engine 244which powers shaft 117 through gear box 246, as shown in FIG. 20.Feedback is based on the data received at base station 146 over wirelessradio link 250 from the wireless torque and angular velocity sensingnode 115 on shaft 117. Motor controller can either switch off electricmotor or engine 244 or it can send a signal to change its speed. Analogoutput base stations are commercially available from Microstrain, Inc inWilliston, Vt. They are called MicroTxRx Wireless Base Station withAnalog Output, and further description of them is available from amanual available from MicroStrain, Inc., “Analog Out Base station QuickStart Guide Rev B,” incorporated herein by reference.

In one example, if data is received at base station 146 shows that shaft117 is shaking violently, data about vibration sensed by anaccelerometer mounted on shaft 117 can be fed back through transmitter142 to analog output 240 of base station 146, and the machine drivingshaft 117 can be turned off, or its speed reduced, to avoid damage toshaft 117 and down time for repairs.

In another example, presently motor current is frequently monitored todetermine an end point to processing with a machine, such as polishingmachine, by detecting a change in current, when polishing removes onelayer of material and starts polishing a different material with adifferent hardness. However, motor current may vary widely during such apolishing operation. Also motor current is only an indirect indicator oftorque in the shaft driving the polishing tool. One embodiment allows adirect measurement of this torque, or another relevant mechanicalproperty of the shaft, such as strain, and this data is transmitted fromshaft 117 and received at base station 146 and provided as an analogoutput for use adjusting operation of motor 244 driving shaft 117. Bydirectly measuring shaft mechanical properties and feeding back data todriving motor 244, greater accuracy in controlling torque in drivingshaft 117 can be provided and such problems as over polishing or metalfatigue in drive shaft 117 from over-driving can be prevented.

While several embodiments, together with modifications thereof, havebeen described in detail herein and illustrated in the accompanyingdrawings, it will be evident that various further modifications arepossible. Nothing in the above specification is intended to limit theinvention more narrowly than the appended claims. The examples given areintended only to be illustrative rather than exclusive.

What is claimed is:
 1. A method of operating a rotatable part,comprising: a. providing a system including a rotatable part, a magnet,an emf generating circuit, a processor, and a transmitter, wherein saidmagnet and said emf generating circuit are mounted for relativerotational motion there between while said rotatable part is rotating,wherein said processor is connected to receive a signal derived fromsaid emf generating circuit and provide data for transmission by saidtransmitter; b. rotating said rotatable part; c. generating an emf insaid emf generating circuit while said rotatable part rotates; d. usinga signal derived from said emf to acquire data indicating a change in amechanical property of said rotatable part while said rotatable part isrotating, wherein said data indicating said change in said mechanicalproperty indicates a condition for maintenance; e. transmittinginformation related to said data with said transmitter; and f. providingmaintenance to said system based on said information.
 2. A method asrecited in claim 2, wherein said rotatable part is on a machine fordelivering torque.
 3. A method as recited in claim 2, wherein saidmachine is one from the group consisting of a motor, a vehicle, a pump,a valve, a compressor, a polishing machine, a grinder, a lathe, or amilling machine.
 4. A method as recited in claim 1, wherein saidprocessor has a sleep mode and an awake mode, further comprising wakingsaid processor from said sleep mode at a frequency in the range fromabout 1 Hz to about 1 kHz.
 5. A method as recited in claim 1, furthercomprising using energy provided by said emf generating circuit to powerat least one component from the group consisting of said processor andsaid transmitter.
 6. A method as recited in claim 1, wherein saidprocessor further comprises controlling power to said transmitter,wherein said processor provides power to said transmitter only duringsaid active mode time period.
 7. A method as recited in claim 1, furthercomprising a sensor mounted for sensing a parameter of said rotatablepart, wherein said sensor comprises at least one from the groupconsisting of a temperature sensor, a strain gauge, a torque sensingdevice, and an accelerometer.
 8. A method as recited in claim 7, furthercomprising sampling said sensor, wherein said sampling said sensorcomprises sampling at a frequency in the range from about 1 Hz to about1 kHz.
 9. A method as recited in claim 7, wherein said processor furthercomprises controlling power to said sensor and wherein said processorsends a signal to provide power to said sensor only during an activemode time period when said processor is awake.
 10. A method as recitedin claim 7, further comprising sensor signal conditioning, wherein saidprocessor further comprises controlling power to said sensor signalconditioning, wherein said processor provides power to said sensorsignal conditioning only during said active mode time period.
 11. Amethod as recited in claim 10, further comprising a sensor signalconditioning power supply for powering said sensor signal conditioning,wherein said sensor signal conditioning power supply comprises an enablepin, wherein said processor further comprises controlling operation ofsaid sensor signal conditioning power supply by sending a signal to saidenable pin.
 12. A method as recited in claim 11, further comprising atransmitter power supply, wherein said processor has separate controlsto enable said sensor signal conditioning power supply and to enablesaid transmitter power supply, wherein said processor further comprisesenabling said signal conditioning power supply independently of enablingsaid transmitter power supply.
 13. A method as recited in claim 12,further comprising a sensor power supply, wherein said processor has anadditional separate control to enable said sensor power supply whereinsaid processor further comprises independently enabling said sensorpower supply.
 14. A method as recited in claim 7, wherein said sensorincludes a strain gauge, wherein said processor further comprisescomputing strain of said rotatable part.
 15. A method as recited inclaim 7, wherein said processor further comprises computing torque ofsaid rotatable part.
 16. A method as recited in claim 1, wherein saidprocessor further comprises computing at least one from the groupconsisting of angular velocity and RPM of said rotatable part.
 17. Amethod as recited in claim 1, wherein said processor further comprisescomputing instantaneous mechanical power of said rotatable part.
 18. Amethod as recited in claim 1, further comprising a receiver, whereinsaid receiver is connected to provide an output to said processor.
 19. Amethod as recited in claim 18, wherein said processor has a sleep mode,further comprising receiving a command through said receiver to wake upsaid processor from said sleep mode.
 20. A method as recited in claim19, further comprising receiving a command to perform at least onefunction from the group consisting of logging said data, transmittingsaid information, and returning said processor to said sleep mode.
 21. Amethod as recited in claim 1, further comprising providing power to saidprocessor and to said transmitter to transmit said information at presettime intervals.
 22. A method as recited in claim 21, further comprisingan energy storage device mounted to provide said power.
 23. A method asrecited in claim 21, wherein said emf generating circuit is connected toprovide power to recharge said energy storage device.
 24. A method asrecited in claim 23, wherein said emf generating circuit comprises aninductor for harvesting energy from relative rotation of said magnet andsaid emf generating circuit.
 25. A method as recited in claim 24,further comprising providing a circuit for converting alternatingcurrent to direct current that is connected to receive electricityderived from said emf generating circuit, providing said processor andsaid transmitter connected for receiving said direct current, drawingsaid direct current to said processor to provide said processor in anactive mode, and drawing said direct current to said transmitter andusing said transmitter for transmitting data.
 26. A method as recited inclaim 23, further comprising providing a circuit for measuring state ofcharge of said energy storage device, said method further comprisingproviding a first measurement of said state of charge of said energystorage device.
 27. A method as recited in claim 26, further comprisingproviding a second measurement of said state of charge of said energystorage device and comparing said second measurement to said firstmeasurement to determine if charge stored in said energy storage deviceis depleting or charging.
 28. A method as recited in claim 27, furthercomprising a sensor mounted for sensing a parameter, further comprisingusing said first measurement of state of charge in said processor todetermine one of: rate of sampling data from said sensor, how long tosample data from said sensor, time between samplings of data from saidsensor, how long to keep said processor in sleep mode, when to awakensaid processor, how long to keep said processor awake, time forproviding power to said sensor, and time for providing power to saidtransmitter.
 29. A method as recited in claim 1, further comprising anon-volatile memory.
 30. A method as recited in claim 29, wherein saidnon-volatile memory includes a unique address to identify datatransmitted.
 31. A method as recited in claim 29, further comprising asensor mounted for sensing a parameter, further comprising storing dataderived from said sensor in said non-volatile memory for transmitting bysaid transmitter at a later time.
 32. A method as recited in claim 1,further comprising adjusting operation of said rotatable part based onsaid data.
 33. A method as recited in claim 1, further comprising amachine and a base station, wherein said machine drives said rotatablepart, wherein said base station is positioned for receiving datatransmitted by said transmitter, wherein said base station includes anoutput to control said machine driving said rotatable part.
 34. A methodof operating a rotatable part, comprising: a. providing a systemincluding a rotatable part, a magnet, an emf generating circuit, aprocessor, and a transmitter, wherein said magnet and said emfgenerating circuit are mounted for relative rotational motion therebetween while said rotatable part is rotating, wherein said processor isconnected to receive a signal derived from said emf generating circuitand provide data for transmission by said transmitter; b. rotating saidrotatable part; c. generating an emf in said emf generating circuitwhile said rotatable part is rotating; d. using a signal derived fromsaid emf to acquire data indicating a change in a mechanical property ofsaid rotatable part while said rotatable part is rotating, wherein saidchange in said mechanical property is related to operation of saidrotatable part; e. transmitting information related to said data withsaid transmitter; and f. adjusting operation of said rotatable partbased on said information.
 35. A method as recited in claim 34, furthercomprising using energy provided by said emf generating circuit to powerat least one from the group consisting of said processor and saidtransmitter.
 36. A method of operating a rotatable part, comprising: a.providing a rotatable part, a magnet, an emf generating circuit, acircuit for converting an alternating current into a direct current, aprocessor, and a transmitter, wherein said processor has an active stateand a sleep mode, wherein said magnet and said emf generating circuitare mounted for relative rotational motion there between while saidrotatable part is rotating for generating an emf in said emf generatingcircuit, wherein said circuit for converting an alternating current to adirect current is connected to receive an alternating current derivedfrom said emf, and wherein said processor and said transmitter areconnected for receiving said direct current; b. rotating said shaft; c.generating said emf, converting to said direct current, waking saidprocessor, and drawing said direct current to said processor to providesaid processor in said active mode; d. drawing said direct current tosaid transmitter and using said transmitter for transmitting data; e.shutting down power to said transmitter; and f. returning said processorto said sleep mode.
 37. A method as recited in claim 36, wherein saiddata is regarding a parameter of said rotatable part.
 38. A method asrecited in claim 36, wherein said data is derived from said emf.
 39. Amethod as recited in claim 36, wherein said processor is connected toreceive a signal derived from said emf and provide data for transmissionby said transmitter.
 40. A method as recited in claim 36, furthercomprising providing at least one from the group consisting ofmaintenance based on said information and adjusting operation of saidrotatable part based on said information.